The present invention relates to ink jet printers and, more particularly, to circuitry for compensation of cross talk between adjacent charging electrodes in a printing system having multiple jets.
Ink jet printers have been developed in which a plurality of jets of ink drops are projected toward a moving print web. Drops in each of the jets are selectively charged. An electric field is provided in the path of the jets such that the charged drops are displaced laterally and directed toward a catching device. The uncharged drops are unaffected by the field and pass through, striking the print web. Typical prior art devices of this type are shown in U.S. Pat. No. 3,373,437, issued Mar. 12, 1968, to Sweet et al. and in U.S. Pat. No. 3,604,980, issued Sept. 14, 1971, to Robertson. The lateral accelerating force applied to the charged drops in such a system is proportional to the magnitude of the applied field and to the magnitude of the charge impressed upon the drops.
Although some ink jet printers print with charged drops, printing with uncharged drops offers several advantages. When printing with uncharged drops, the print trajectory of the drops is straight. It is therefore not difficult to determine the point of impact of a drop on the web. Additionally, if printing were done with charged drops, the print positions on the web could be a function of both the deflection field and the charges on the drops. Slight fluctuation in the charges induced on the drops would, therefore, result in deterioration of the image produced.
In systems in which printing is accomplished with uncharged drops, it is important that the print drops do not receive a slight charge inadvertently. One source of inadvertent charging is the drops which have been previously formed in the stream. Assuming the previous drop in a stream carries a charge, the subsequent uncharged drop will be formed in sufficient proximity to the charged drop that a slight charge of opposite polarity may be induced. Such drop-to-drop interference has been recognized as a significant problem and has been treated in several patents, such as U.S. Pat. No. 3,828,354, issued Aug. 6, 1974 to Hilton; U.S. Pat. No. 3,512,173, issued May 12, 1970, to Damouth; U.S Pat. No. 3,827,057, issued July 30, 1974, to Bischoff et al.; U.S. Pat. No. 3,789,422, issued Jan. 29, 1974, to Haskell et al.; U.S. Pat. No. 3,833,910, issued Sept. 3, 1974, to Chen; and, U.S. Pat. No. 3,631,511, issued Dec. 28, 1971, to Keur.
In U.S. Pat. No. 3,656,171, issued Apr. 11, 1972, to Robertson, the problem of cross talk between adjacent jets was also recognized. Typically, the drops in an individual jet are selectively charged by a charge electrode which is positioned adjacent the point at which the drops are formed. When the electrode is charged, an opposite charge is induced in the drop as it is being formed and this charge remains on the drop.
In a system in which the jets are positioned relatively close together to increase the system resolution a charge may be induced in a drop by a charging electrode associated with an adjacent jet. The effect of a charge so induced upon a charged drop, which is to be caught, is merely to alter its deflected trajectory slightly; the catching arrangement is such, however, that such a drop will still be caught. A print drop which is inadvertently charged by an adjacent charge electrode will be slightly deflected into a trajectory which will significantly affect the printing resolution of the system, however. While the Robertson U.S. Pat. No. 3,631,511 recognizes the problem of cross talk, the nature of the device disclosed therein is such that the effect of such cross talk is minimized and no compensation is needed. Where an externally generated deflection field is used for deflection, however, the cross talk effect will be more substantial and it is desirable to be able to compensate for such cross talk.
In U.S. Pat. No. 3,604,980, issued Sept. 14, 1971, to Robertson, inter-jet or inter-channel cross talk was also recognized as presenting a problem, with the suggested solution being an increase in shielding between charging electrodes. As the distance between jets is reduced, however, shielding becomes less feasible. A need exists, therefore, for a circuit arrangement which will minimize the effect of cross talk from adjacent charging electrodes.